A. Field of Invention
This invention pertains to the field of cardiovascular biomedical engineering. More specifically, this invention pertains to the field of indirect blood pressure measurement using a direct pressure sensing transducer, such as a piezoelectric ceramic.
B. Description of Prior Art
1. Problem Statement/Objects of Invention
Ideally, for a blood pressure instrument to be technically and economically suitable for extended periods of time and during most normal daily activities, it should be accurate, repeatable, easy to manufacture, simple in structure, inexpensive, easy to use, give beat-to-beat (continuous) pressure data, be comfortable to the patient, be low in power consumption, be small in size, have very few environmental restrictions on its usage, be easily applied to the patient, be easy to calibrate, be reliable, have low sensitivity to motion artifact effects, measure a strong signal, (to maximize the signal-to-noise ratio), and measure pressure as directly as possible (to maximize blood pressure data correlation and minimize secondary effect correlations that cause errors with indirect methods).
2. Prior Art Performance
Direct, invasive prior art has many positive points: they are generally accurate, repeatable, continuous (they give beat-to-beat data), relatively easy to calibrate (they use a 2 point calibration method as does the instant invention, reliable, measure a strong signal, and measure pressure directly. Their negative points are: they are difficult to manufacture, expensive, difficult to use, uncomfortable to the patient (tubes are put directly into the bloodstream), high in power consumption, relatively large in size, are very restricted in terms of the environments in which they can be used (motion artifact is easily caused, patients can be injured, and invasive techniques are highly susceptible to infections), difficult to attach to the patient, and highly sensitive to motion artifact (thus taking away their accuracy and repeatability positive points in an ambulatory environment).
Indirect, noninvasive prior art has many positive points: some of them are accurate (e.g. many arm cuff units utilizing the auscultatory and oscillometric methods work very well if the patient remains motionless), repeatable, easy to manufacture, simple in structure, the simplist units are inexpensive, easy to use, easily applied to the patient, easy to calibrate, reliable, and are more comfortable than invasive prior art. Their negative points are: the instruments that work can only give readings once every minute or so (i.e. they are not continuous), the arm cuff (working) units are very uncomfortable and even dangerous to the patient if used for more than about a minute at a time, long term usage units are expensive, they are typically high in power consumption (they typically have to power pneumatic machines such as pumps, solenoids, and valves), automated units are large in size (alot of this is due to the pneumatic machinery and power supply taking up much volume), they have many environmental restrictions on their usage (e.g. patient motion, temperature, ambient sound levels, etc), they have high sensitivity to motion artifact effects, they measure weak signals, and they don't measure the blood pressure directly (e.g. they detect blood flows as indirect indicators that blood has gotten through an occluding cuff's pressure seal, they identify diastolic pressure when they can no longer detect the sounds of the blood flow fighting against the occluding cuff, etc).
As a further note, the specific problem with all of the prior art blood pressure devices that try to use the finger appears to be their practicality. Their practicality (i.e. usefulness) is questioned because of their inherent problems with accuracy, repeatability, reliability, and calibration consistency. Their lack of market strength is attributed largely to this lack of practicality.
Note also that much of the practical problems illustrated by prior art finger blood pressure devices is that they did not give the patient's heart level pressures. Hand height errors are typically 0.75 mmHg of pressure error per cm of hand to heart height difference. Compensation for this height problem must be made if the advantages of using the finger over the arm are to be realized (advantages include patient safety, patient comfort, very low power consumption, and small size).
Documentation to support the above prior art description can be found in the following publications:
1. U.S. Pat. Nos.: 2,452,799 (capacitive transducer in a cuff to measure volume changes); 2,755,796 (capacitive transducers); 3,039,044 (electromagnetic pressure transducer); 3,099,262 (fluid pressure sensor); 3,102,534 (fluid pressure sensor); 3,107,664 (piezoelectric pulse sensor); 3,123,068 (sphygmomanometer); 3,132,643 (BP measurement using ECG to pulse times); 3,176,681 (piezoelectric pulse sensor); 3,219,035 (strain guage direct pressure transducer); 3,228,391 (pulse rate transducer); 3,229,685 (optical systolic measurement); 3,230,950 (indirect BP determination); 3,400,709 (strain gauge pressure transducer circuit with minimum and maximum sample and hold characteristics); 3,482,565 (electromagnetic); 3,486,499 (a circuit that outputs the minimum, maximum, and average voltage from a typical strain gauge type transducer); 3,573,394 (piezoelectric microphone); 3,585,987 (finger cuff pressure tracking system for systolic pressure); 3,661,146 (piezoelectric for flow measurement); 3,704,708 (strain-type pressure transducer casing); 3,769,964 (inflation/deflation of a fluid-filled cuff to identify systolic and diastolic pressures); 3,835,839 (impedence plethysmograph for flow); 3,880,145 (2 force transducer blood pressure device); 3,894,535 (direct pressure system zeroing); 3,920,004 (piezoelectric element to indicate pulse occurrance for systolic with a cuff); 3,996,927 (hand/heart height compensation table); 4,030,484 (strain sensor); 4,030,485 (optical, relative systolic); 4,038,976 (piezoelectric pulse sensor); 4,074,710(1) (inflating and deflating cuff until systolic is found); 4,105,021 (watching pulses in cuff pressure to determine pressures); 4,127,114 (piezoelectric doppler); 4,137,907 (looking at systolic rise rate acceptance window on pressure wave); 4,141,346 (ocular plethysmograph); 4,144,879 (pressure tracking system and waveform identification method); 4,161,173 (invasive blood pressure gauge); 4,172,450 (servo controlled cuff pressure to track pressures); 4,177,801 (oscillometric method); 4,185,621 (display and battery charger unit using a piezoelectric to detect pressure); 4,269,193 (strain-gauge type noninvasive pressure transducer); 4,307,727 (wrist band transducer support apparatus; using the radial artery); 4,423,738 (strain-gauge pressure transducer); 4,425,920 (pulse transit time and impedence plethysmography); 4,338,950 (accelerometer mounted on the wrist to detect motion of the wrist); 4,465,075 (IC pressure transducer).
2. Foreign patents: 2306444 (Technological Service, Inc, 8/73. Country DT) (detects temporal pulses by pressing a pneumatic cuff against the temporal artery using a finger).
3. Other sources:
G. Francis. An Improved Systolic-Diastolic Pulse Separator. Medical and Biological Engineering. Jan. 1974. (a circuit that finds the minimum and maximum points of each input waveform). PA0 W. Naylor. An Analog Preprocessor for Use in Monitoring Arterial Pressure. Biomedical Engineering. Feb. 1971. (circuit for finding the minimum, maximum, and average values off of varying signals).
3. This Invention's Performance
This invention is different and better than the prior art because it solves all of the problems listed earlier.